Valves are used in a variety of industries to control the flow of fluids, in particular in the oil and gas industry. Most valves provide a valve body having a longitudinal flow bore and a transverse gate cavity that intersects the flow bore. Actuating devices are used particularly for moving a closing plate of gate cavity between a closed position, in which the plate obstructed the flow bore, and an open position, in which the plate does not obstruct the flow bore. Actuating devices are driven by an input torque, the necessary torques being generally produced with handwheels or with electromechanical wheels.
Actuating devices further provide transmission mechanism able to transform a rotation motion into a translation motion, being provided with a translating shaft that drives the opening and closing of the gate plate and a rotating sleeve connected to the torque wheel.
As it has been observed, the force to apply to the translating shaft of the actuating device is maximum at the valve opening and in a first opening run. Then the force is less important and substantially constant. On the contrary, when the valve has to be closed, the maximum force is to be exerted at the valve final closing. When using a handwheel, it happens that the force is so important at the start of the valve opening and the end of the valve closing that it may require two operators to exert the requested force on the handwheel.
Known actuating devices including double screw connecting means are described in the patent EP-A-1 359 345.
Such actuating devices permit to reduce the necessary torque at the start of the valve opening since the second screw mechanism of higher efficiency is first engaged. The end of the opening run is realized by the first screw mechanism.
However, the start of the valve closing is also realized by the second screw mechanism of higher efficiency being first engaged. The end of the closing run is then realized by the first screw mechanism. The end of the valve closing run requires the maximum of exerted force but, with such an arrangement, it is the screw mechanism with the lowest efficiency that is used.